About one quarter of the world's population is exposed to the risk of malaria and more than a million people die of malaria each year. Of the four species of malarial parasites that infect humans, the two major species are Plasmodium falciparum and P. vivax. 
The P. falciparum blood stage merozoites bind to and parasitize the erythrocytes using a variety of surface proteins (Cowman et al., FEBS Lett. 476:84-88 (2000); Baum et al., J. Biol. Chem. 281:5197-5208 (2006)), a major antigenic member of which is called Merozoite Surface Protein 1 (MSP1), a 195 kDa protein. MSP1 is present in all the erythrocyte-invasive species of Plasmodium, anchored to the merozoite surface by a glycosyl-phosphatidylinositol linkage. During early stages of the erythrocyte invasion process, soon after release from infected erythrocytes, the merozoite MSP1 protein undergoes proteolytic cleavage, producing a C-terminal cleavage product MSP1-42, which subsequently undergoes a second cleavage, producing an 11 kDa peptide MSP1-19, which remains attached to the parasite surface as it enters the erythrocyte. The formation of the cleavage product MSP1-19 is very important for successful invasion by the parasite since inhibition of its proteolytic formation or its neutralization by monoclonal antibodies prevents entry of the parasite to the erythrocytes (Blackman et al., J. Exptl., Med. 180:389-393 (1994)).
The MSP1-19 peptide is one of the most important malaria vaccine candidates available. MSP1-19-specific antibodies from malaria-resistant human sera react with the antigen and include a major erythrocyte-invasion inhibitory component (Holder & Riley, Parasitol. Today, 12: 173-174 (1996); O'Donnell et al., J. Expt. Med. 193: 1403-1412 (2001)). Serum from donors in malaria-endemic regions usually demonstrates strong antibody reactivity towards Pf MSP1-19. (Nwuba et al., Infect. Immun. 70: 5328-5331 (2002))
The monoclonal antibody (mAb) G17.12 was raised against recombinant Pf MSP1-19 and recognizes its epitope on the parasite surface, demonstrating that this region of the antigen is accessible on the native MSP1 polypeptide complex (Pizarro et al., J. Mol. Biol. 328:1091-1103 (2003)). Interestingly, erythrocyte invasion experiments in vitro showed that infection is not inhibited in the presence of G17.12, even at 200 μg/ml concentration and G17.12 does not inhibit in vitro secondary processing of MSP1. Id. The presence of antibodies that block the binding of invasion-inhibitory antibodies, thereby facilitating parasite survival, has also been demonstrated (Guevara Patino et al., J. Expt. Med. 186: 1689-1699 (1997)), and may be responsible for the failure of G17.12 mAb to inhibit erythrocyte invasion by M. falciparum. 
Cerebral malaria, a rare but fatal infection restricted to P. falciparum invasion of brain capillaries because of the sequestration of parasitized erythrocytes, is often untreatable because most drugs cannot cross the blood-brain barrier to reach the brain capillaries. Adhesion of P. falciparum—infected erythrocytes to brain capillaries is mediated by the interaction of parasite ligands Pf Emp-1 family of proteins expressed on the surface of infected erythrocytes with ICAM-1 and CD36 expressed on the surface of capillary endothelium cells in cerebral vessels. (Smith et al., Proc. Natl. Acad. Sci. USA 97:1766-1771 (2000); Franke-Fayard et al., Proc. Natl. Acad. Sci. USA 102, 11468-11473 (2005))
Although a few drugs, such as chloroquine that targets the heme detoxification pathway, are used to treat malaria, there are increasing incidence of parasite resistance to drugs and mosquito vector resistance to insecticides. Chloroquine antagonizes heme polymerization mediated by parasite-induced HRPs (histidine-rich proteins), as heme monomers are highly toxic for malaria parasites. The polymerization of heme allows detoxification, which is reversed by chloroquine. Another drug, artemisinin, is effective against chloroquine-resistant P. falciparum in cerebral malaria. Artemisinin forms adducts with globin-bound heme in hemoglobin, which binds HRPs to prevent heme polymerization. There is an urgent need to find new drugs for this dreaded disease that is particularly prevalent in Africa and Asia. Present attempts at drug development are directed towards deciphering the complete parasite genome sequence, molecular modeling of the malaria parasite proteins and a search for novel drug targets.